JP7263654B2 - optical security element - Google Patents

Description

The present invention relates to the technical field of reflective or refractive optical security elements operable to project caustic patterns under suitable illumination, and methods for designing such optical security elements.

There is a need for security features on objects that allow a so-called "common man" to determine authenticity using commonly available means. These means include the use of the five senses, primarily visual and tactile, as well as the use of prevalent tools such as mobile phones.

Some common examples of security features are forensic fibers, threads or foils (embedded in substrates such as paper), watermarks, intaglio or microprinting (possibly optically variable ink are printed on substrates using ), and they can be found on banknotes, credit cards, IDs, tickets, certificates, documents, passports, and the like. These security features may include optically variable inks, transparent inks, or luminescent inks (fluorescent or phosphorescent under appropriate illumination with specific excitation light), holograms, and/or tactile features. can also The main aspect of a security feature is that it has some physical property (optical effect, magnetic effect, material structure, or chemical composition) that is very difficult to counterfeit, and such a security feature is used to mark An object marked with can be reliably considered authentic if its properties can be observed or revealed (either visually or by means of a particular device).

However, when the object is transparent or partially transparent, these functions may not be suitable. Indeed, transparent objects often require security elements with the required security function not to change the transparency or appearance of the object aesthetically or functionally. Notable examples include blisters and vials for pharmaceuticals. In recent years, for example, polymer and hybrid banknotes have incorporated transparent windows in their design, thus creating a desire for security features compatible with the windows.

Most of the existing security features of security elements for documents, banknotes, secured tickets, passports, etc. were not specifically developed for transparent objects/areas and are therefore not well suited for such applications. Not suitable. Other functions, such as those obtained using transparent and fluorescent inks, require specific excitation and/or detection tools, which may not be readily available to the "general public". be.

Translucent optically variable features (eg, liquid crystal coatings or latent images from surface structures) are known and can provide such features. Unfortunately, marks incorporating such security features generally must be viewed against a dark/uniform background for the effect to be well visible.

Other known features are diffractive optical elements such as non-metallized surface holograms. The drawback of these features is that they exhibit a very low contrast visual effect when viewed directly. Furthermore, when used in combination with a monochromatic light source to project a pattern, lasers are typically required to obtain satisfactory results. Moreover, a fairly precise relative spatial arrangement of the light source, the diffractive optical element, and the user's eye is required to provide a clearly visible optical effect.

For example, laser-engraved micro-letters and/or micro-encodings have been used, for example, on glass vials. However, they require expensive tools for implementation and specific magnification tools for detection.

It is therefore an object of the present invention to provide an optical security element for transparent or partially transparent objects (or substrates), without further means (i.e. with the naked eye) or generally easily accessible. It is an object of the present invention to provide an optical security element having a security function that can be easily authenticated visually by a person using available means (for example, a simple magnifying glass). Another goal of the present invention is to provide an optical security element that is easily mass produced or compatible with mass production manufacturing processes. Furthermore, the illumination of the optical security element should also be possible using readily available means (e.g. light sources such as LEDs in mobile phones, or sunlight) and conditions for good visual observation by the user. However, it should not require an overly strict relative spatial arrangement of the light source, the optical security element and the user's eye.

Moreover, most of the objects listed above have a smaller size in at least one dimension (eg banknotes can be only less than 100 μm thick). It is therefore a further object of the present invention to provide an optical security element that is compatible with objects having smaller dimensions (eg thickness less than 300 μm).

A further object of the present invention is to provide an efficient method for selecting a target visual effect that conforms to the reduced size(s) mentioned above.

According to one aspect, the present invention is an optical security element comprising a reflective or a transparent or partially transparent light deflecting surface that deflects and projects incident light from a light source. having a relief pattern operable to form an image on a projection plane, the optical parameters of this optical security element meeting certain projection criteria, the projected image being projected without further means (i.e. with the naked eye) or in general a caustic pattern that reproduces a reference pattern readily recognizable by humans using means readily available to the public, so that an object marked with this optical security element can be easily visually authenticated by a person. can be an optical security element. Due to the reduced thickness of the relief pattern of the optical security element, this optical security element marks, for example, objects having smaller dimensions, such as banknotes or security documents (such as identification cards, passports, cards, etc.). will be particularly suitable for Due to the transparent aspect of the refractive optical security element, the refractive optical security element can be applied to at least partially transparent substrates (e.g. glass or plastic bottles, bottle caps, watch glasses, jewelry, gemstones, etc.). ) will be particularly suitable for marking

Especially when the relief pattern of the optical security element is very thin (i.e. typically has a relief depth of less than 250 μm), it can be conveniently reproduced on the projection plane by the projected caustic pattern and is visually readable by humans. Given that it is very difficult to determine a reference pattern that will be visually recognizable, another aspect of the invention is to identify a reference pattern to be reproduced by a projected caustic pattern according to a specific digital image selection test. It relates to a method for efficiently designing a relief pattern of a light deflection surface of an optical security element based on selection of candidate digital images of the pattern. If the candidate digital image complies with the test requirements, calculate the corresponding relief pattern with the specified depth and then the reflective light deflecting surface, or transparent or partially transparent light deflecting surface of the specified index of refraction. A selected digital image that has been processed to reproduce the calculated relief pattern and reaches the optical security element that satisfies the projection criteria described above and is readily visually perceptible to humans under proper lighting. It is possible to obtain an optical security element that gives a projected caustic pattern that reproduces the reference pattern of . This method is particularly efficient for designing very thin relief patterns (i.e., 250 μm or less, or even 30 μm or less depth) that are convenient for visual authenticity of marked objects, allowing the design process Allows to significantly accelerate operations.

Thus, according to one aspect, the present invention is an optical security element comprising a reflective light-deflecting surface or a refractive transparent or partially transparent light-deflecting surface of refractive index n, wherein from the light deflecting surface A deep light source configured to deflect incident light received from a point light source at a distance d _s and form a projected image including a caustic pattern on a projection plane disposed at a distance d _i from the light deflection plane. with a relief pattern of height δ, said caustic pattern reproducing the reference pattern, an optical security element by illumination of an area of value A of the relief pattern by a light source and an optical security element onto the projection plane (average) Upon delivery of the illuminance value E _A , the average illumination value E _α1 over the circular area of the selected value α ₁ within the region of the projected image at the projection plane is calculated according to the projection criterion E _α1 ≤ E _A (1/2+α ₀ /α ₁ +√(1/4+α ₀ /α ₁ )) and the scaling area parameter α ₀ =4πd _i δ for reflective light deflecting surfaces or α ₀ =2π(n -1) for optical security elements with d _i and α ₁ smaller than the area value A;

The value _of d _i should be 30 cm or less, and the value of _{the ratio} should preferably be at least 5 or more. Also, the projection plane is preferably flat.

In order to provide a very thin optical security element, the value of the depth δ of the relief pattern can be 250 μm or less, or even 30 μm or less. Furthermore, the optical security element may further comprise a relief pattern disposed on the flat base of the optical material substrate, the total thickness of the optical security element being 100 μm or less.

According to another aspect, the invention is for designing a relief pattern with a depth less than or equal to the value δ of a reflective light-deflecting surface or a refractive transparent or partially transparent light-deflecting surface of refractive index n. wherein the relief pattern deflects incident light received from a point light source at a distance d _s from the light deflection surface and the projected image comprising the caustic pattern is disposed at a distance d _i from the light deflection surface. in the region of the projected image at the projection plane upon illumination of an area of value A of the relief pattern by the light source and delivery of illuminance value _EA by the optical security element to the projection plane satisfies the projection criterion E _α1 ≦E _A (1/2+α ₀ /α ₁ +√(1/ _{4 +} α ₀ /α ₁ )) and the reflectivity or α ₀ =2π(n−1) _{d i for} refractive light deflecting surfaces where α ₁ is larger than _the area value A small,
a) selecting a digital image of the reference pattern to be reproduced on the projection plane by the caustic pattern, the digital image having a total number of pixels N _A and the sum of all pixel values across the digital image being I _A and for each circular region of N (N is an integer, 1≤N≤N _A ) pixels in the digital image, the value I(N ) is less than the value I _max (N)=N(I _A /N _A )(1/2+N ₀ /N+√(1/4+N ₀ /N)), N ₀ is the number of pixels given by N _A (α ₀ /A) in the digital image;
b) calculating a relief pattern of depth no greater than δ corresponding to the reference pattern in the digital image selected in step a);
c) processing the surface of the optical material substrate to form a light deflection surface that reproduces the relief pattern calculated in step b) to obtain an optical security element comprising the processed light deflection surface. Regarding the method.

This method tests for I(N)<I _max (N) by adapting the pixel values as necessary to fully comply with the test for any N with 1≦N≦ _NA ( or selection criteria), or only partially (i.e., satisfying only a partial circular region of the N pixels). Accordingly, step a) of selecting digital images of reference patterns includes the further step of modifying candidate digital images of reference patterns, some of which do not satisfy the selection criterion that I(N) is less than _Imax (N). and the further step may be performed by applying adapted pixel values to that portion of the candidate digital image to comply with the selection criteria for arbitrary N, 1≤N≤N _A . This is done by adapting the pixel values within that portion of the image to provide a modified candidate digital image from which to select. Adaptation of pixel values may also be obtained by a filtering operation. Thus, the pixel values of the candidate digital image can be adapted by filtering the candidate image with a filter (e.g., a high-pass filter) to reduce the contrast of the image, and the parameters of the filter (e.g., the cutoff of the high-pass filter) frequency) corresponds to the selection criteria.

Thus, according to this variant of the invention, an inappropriate target pattern represented in the digital image is transformed into an appropriate one with respect to the digital image selection criteria of the invention, which can be selected in subsequent step a). Is possible.

In step c) of the method, processing the surface of the optical material substrate may comprise any one of ultra-precision machining (UPM), laser ablation, and lithography.

A light deflecting surface processed by the method can be a master light deflecting surface used to construct a replica (or a mass-produced replica of an optical security element) by molding techniques (e.g. can be replicated on a substrate) to form an applicable mark. Fabricated light deflecting surface replication may involve any one of UV casting and embossing (eg, roll-to-roll or foil-to-foil manufacturing processes).

According to a further aspect, the invention is a method of visually authenticating by a user an object marked with an optical security element according to the invention, comprising:
illuminating the light deflection surface of the optical security element with a point light source at (approximately) a distance _ds from the light deflection surface;
visually observing the projected caustic pattern on a projection plane at a distance d _i from the optical security element;
determining that an object is authentic when a user assesses that the projected caustic pattern is visually similar to the reference pattern.

The invention will now be described in more detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, like numerals represent like elements throughout the different drawings, the salient aspects and features of the present invention are illustrated.

1 is a schematic diagram of an optical configuration of a refractive optical element for projecting a caustic pattern according to a preferred embodiment of the invention; FIG. FIG. 10 illustrates a reference pattern in a candidate digital image representing the number 100; 3 illustrates the selection of a digital image of the reference pattern of FIG. 2 in accordance with the present invention and shows the results of scanning the candidate digital image of FIG. 2 with different scan windows; FIG. 3 illustrates the selection of a digital image of the reference pattern of FIG. 2 in accordance with the present invention and shows the results of scanning the candidate digital image of FIG. 2 with different scan windows; FIG. 3 illustrates the selection of a digital image of the reference pattern of FIG. 2 in accordance with the present invention and shows the results of scanning the candidate digital image of FIG. 2 with different scan windows; FIG. 3 illustrates the selection of a digital image of the reference pattern of FIG. 2 in accordance with the present invention and shows the results of scanning the candidate digital image of FIG. 2 with different scan windows; FIG. 3 illustrates the selection of a digital image of the reference pattern of FIG. 2 in accordance with the present invention and shows the results of scanning the candidate digital image of FIG. 2 with different scan windows; FIG. FIG. 4 is a diagram showing an example of a reference pattern; FIG. 4A and 4B are further illustrations of the selection of digital images of the reference pattern of FIG. 3 in accordance with the present invention, showing the results of scanning the candidate digital images of FIG. 3 with various scan windows; 4A and 4B are further illustrations of the selection of digital images of the reference pattern of FIG. 3 in accordance with the present invention, showing the results of scanning the candidate digital images of FIG. 3 with various scan windows; 4A and 4B are further illustrations of the selection of digital images of the reference pattern of FIG. 3 in accordance with the present invention, showing the results of scanning the candidate digital images of FIG. 3 with various scan windows; 4A and 4B are further illustrations of the selection of digital images of the reference pattern of FIG. 3 in accordance with the present invention, showing the results of scanning the candidate digital images of FIG. 3 with various scan windows; 4A and 4B are further illustrations of the selection of digital images of the reference pattern of FIG. 3 in accordance with the present invention, showing the results of scanning the candidate digital images of FIG. 3 with various scan windows; Fig. 3 shows a thin transparent refractive optical security element cast on a foil substrate (foreground) and a corresponding projected caustic pattern (background); 4B is a photograph of a caustic pattern projected by the optical security element shown in the foreground of FIG. 4A; FIG. 4 is a projected caustic pattern corresponding to a reference pattern showing a portrait of George Washington;

In optical systems, the term "caustic" refers to the envelope of a ray reflected or refracted by one or more surfaces, at least one of which is curved, and the envelope of such ray on another surface. means projection to More specifically, a caustic is a curve or surface tangential to each ray that defines the boundary of the ray envelope as a dense light curve. For example, the light pattern formed by sunlight on the bottom of a swimming pool is a caustic "image" or pattern formed by a single light-deflecting surface (a wavy air-water interface), whereas the water-filled Light passing through the curved surface of a glass filled with water will see a glass of water placed when it intersects two or more surfaces (e.g., air-glass, glass-water, air-water, etc.) that deflect the light's path. creates a cusp-like pattern on the table on which it is placed.

In the following, the most common configuration in which the (refractive) optical (security) element is defined by one curved surface and one plane is used as an example, without restricting the more general case. Herein, an appropriately shaped (i.e., having an appropriate relief pattern) optical surface deflects light from a light source, deflects light from some areas of the screen, and directs light to other areas of the screen. When light is concentrated in a light pattern (i.e., thereby forming the above-mentioned "caustic pattern"), the more general "caustic pattern" (or "caustic image") is projected onto the screen (projection surface). is called a light pattern formed in Deflection describes the change in the path of a light ray from a light source in the presence of an optical element relative to the path from the light source to the screen in the absence of the optical element. Furthermore, curved optical surfaces are called "relief patterns" and optical elements provided with this surface are called optical security elements. Note that the caustic pattern may be the result of light deflection by multiple curved surfaces and multiple objects, possibly at the cost of increased complexity. Furthermore, relief patterns for producing caustic patterns should not be confused with diffraction patterns (eg security holograms, etc.).

According to the invention, it can be found that this concept can be applied to general objects such as consumer products, ID/credit cards, banknotes, for example. For applications, a significant reduction in the size of the optical security element is required, in particular a sub-permissible relief depth. Surprisingly, the depth of the relief was strongly limited, but of sufficient quality to allow visual recognition of the selected image from the visually observed caustic pattern on the projection plane (or screen). It has been found that it is still possible to achieve an approximation of the selected (digital) image (representing the reference pattern) on the projection plane. Such direct recognition of the reference pattern from the caustic pattern faintly visible on the screen projected from the optical security element which is rather difficult to design and fabricate (and therefore very difficult to counterfeit) is the key to this optical security. The element is used to constitute a useful security test that allows positive authenticity determination of marked objects.

As used herein, "relief" refers to the presence of elevation differences between the highest and lowest points of a surface (i.e., as a "peak-to-valley" scale), such as the difference in elevation between the valley and peak of a mountain (i.e., on a "peak-to-valley" scale). measured along the optical axis of the optical security element). According to a preferred embodiment of the present invention, the maximum depth of the relief pattern of the optical security element is no more than 250 μm or more preferably no more than 30 μm, the limits imposed by ultra-precision machining (UPM) and reproduction processes, i.e. about greater than 0.2 μm. According to this specification, the height difference between the highest point and the lowest point in the relief pattern of the light deflection surface is called a relief depth δ.

Several terms are used herein and are further defined below.

A caustic pattern (image) forming an approximation of a digital image is to be understood as the light pattern projected by the optical security element when illuminated by a suitable light source (preferably, but not necessarily, a point light source). As mentioned above, an optical (security) element is to be understood as a plate of refractive material that contributes to the creation of the caustic image.

The light deflection surface(s) is (one or more) of the optical security elements that contribute to deflecting the incident light from the light source onto the screen or (preferably flat) projection surface on which the caustic pattern is formed. ) surface.

Optical material substrates used for making optical (security) elements are raw material substrates whose surface has been specially processed to have a relief pattern thereby forming a light deflecting surface. For reflective light-deflecting surfaces, the optical material substrate need not be homogeneous or transparent. For example, the material may be opaque to visible light (wherein the reflectivity is provided by conventional metallization of the surface being processed). In the case of a refractive light-deflecting surface, the raw material substrate is transparent (or partially transparent) (for photons in the spectrum visible to the human eye) and homogeneous and has a refractive index n, corresponding to The light deflection surface is referred to as a "refractive transparent or partially transparent light deflection surface of refractive index n".

A master according to this specification is the first physical realization of a light deflection surface from a calculated profile (in particular a calculated relief pattern). A master can be replicated into several copies (tools), which are then used for mass replication.

A point light source, as used herein, considers the light to originate from a single point of sufficiently small angular size (from the point of view of the optical security element) at a distance _ds from the light deflection surface. It is a light source that can As a rule of thumb, this means that the quantity (source diameter)*d _i /d _s is the desired target caustic pattern of the projected image on the projection plane at distance d _i from the light deflection plane (see FIG. 1). It means smaller than the resolution (eg 0.05-0.1 mm). A screen should be understood as a surface onto which a caustic pattern is projected. Also, the distance between the light source and the light deflection surface is called the light source distance _ds , and the distance between the light deflection surface and the screen is called the image distance d _i .

The term "tool" (or "replication tool" when necessary for disambiguation) is primarily used for a physical object having a light deflection surface profile that is used for mass replication. This may for example be to produce a copy of the master surface (from a master with a corresponding inverted relief the original relief is reproduced by embossing or injection). For the tools used to machine the relief pattern of the light deflection surface, the term "fabrication tool" is used for disambiguation.

According to a preferred embodiment of the invention, an optical security element (1) having a reflective or refractive surface, deflecting light from a point light source S and projecting it onto a suitable screen (3). provided. The screen (3) can be any surface (mostly planar), or any object (a flat portion of), etc., on which a meaningful image is formed, as shown in FIG. A special design of the light deflection surface may allow a (recognizable) caustic pattern to be projected onto the curved surface. Images may be, for example, logos, photographs, numbers, or any other information that may be relevant in a particular context. The screen is preferably a flat projection surface.

The configuration of Figure 1 shows that light from a source S is deflected by a suitably shaped optical surface having a relief pattern (2). This general idea is known, for example, from reflective surfaces for automobile headlights, reflectors and lenses for LED lighting, and optical systems in laser optics, projectors and cameras. Usually, however, the goal is to transform a non-homogeneous distribution of light into a homogeneous distribution. In contrast, the goal of the present invention is to obtain a non-homogeneous light pattern, i.e. a caustic pattern, which is relative to the reference pattern (as represented on the (digital) reference image). Reproduce (approximately) some regions with luminance. The illuminated relief pattern (2) of the optical element forms a caustic pattern (4) on the screen (3) to replace the known reference pattern (5) with sufficient quality (possibly an overall intensity scaling factor ), a person visually observing the caustic pattern on the screen can easily ascertain whether it is a valid reproduction of the reference pattern and that the caustic pattern is the reference pattern. If the pattern is sufficiently similar, we consider (almost certainly) the object marked by the optical security element to be authentic.

According to the embodiment of FIG. 1, a ray (6) from a light source S, which according to this example is a point light source, is at a light source distance d _s comprising a light deflection surface with a relief pattern (2) (refracted). Propagate to optical security element (1). The optical security element is here made of a transparent or partially transparent homogeneous material of refractive index n. A so-called caustic pattern (4) is projected onto the screen (3) at an image distance d _i from the light deflection surface of the optical security element (1). The authenticity of the optical security element (and thus the object marked with this security element) can be assessed directly by visually checking the degree of similarity between the projected caustic pattern and the reference pattern. can be done.

The relief pattern (2) is preferably first calculated from the specified target digital image. Methods for such calculations are described, for example, in EP-A-2711745 and EP-A-2963464. From that calculated relief pattern, the corresponding physical relief pattern is produced using ultra-precision machining (UPM) on the surface of a suitable optical material substrate (e.g. transparent or partially transparent material of refractive index n). , or a reflective surface of an opaque material). When a relief is fabricated on the surface of an opaque optical material substrate to form a reflective surface, good reflectivity is obtained by the further conventional operation of depositing (metallization) a thin layer of metal on the relief. UPM uses diamond machining and nanotechnology tools to achieve very high precision so that tolerances can reach "sub-micron" and even nanoscale levels. In contrast, "high precision" in conventional processing implies single digit tolerances in micrometers. Other techniques that may be suitable for creating physical relief patterns on surfaces are laser ablation and grayscale lithography. As is known in the field of microfabrication, each of these techniques has different advantages and limitations in terms of cost, accuracy, speed, resolution, and so on. In general, relief patterns calculated for generating caustic patterns have a smooth profile (ie no discontinuities) with a typical depth of at least 2 mm for an overall size of 10 cm x 10 cm.

Optical material substrates suitable for refractive, light-deflecting optical elements should be optically transmissive, transparent or at least partially transparent, and mechanically stable. Typically, the transmittance T is preferably 50% or more, and most preferably T is 90% or more. Also, a low haze value H of 10% or less can be used, but preferably H is 3% or less, and most preferably H is 1% or less. Also, the optical material should behave correctly to provide a smooth, defect-free surface during the fabrication process. An example of a suitable substrate is an optically transparent PMMA slab (also known by trade names such as Plexiglas, Lucite, Perspex, etc.). For reflective caustic light-deflecting optical elements, suitable optical material substrates should be mechanically stable and should be capable of being given a mirror finish to the substrate. One example of a suitable substrate is a metal, such as those used for ruled grating masters and laser mirrors, or a non-reflective substrate that can be further metallized.

For large-scale production, additional steps of tooling and mass replication of optical security elements onto target objects are required. A suitable process for making tools from a master is, for example, electroforming. Processes suitable for mass replication are, for example, hot embossing of polymer films or UV casting of photopolymers. However, for mass replication, neither the master nor the tools derived from the master need to be optically transparent, so even if the final product is a refractive optical element, an opaque material (particularly metal ) can also be used. Nevertheless, in some cases it may be advantageous for the master to be transparent. This is because it allows checking the quality of the caustic image before tooling and mass duplication.

One important aspect regarding the use of an optical element (comprising a light deflecting surface with a relief pattern) as a security feature is the physical scale of the element, the physical dimensions of which project the target object and the caustic image. must be compatible with the optical configuration required to

Generally, for such applications, the maximum lateral size is limited by the overall size of the object and can usually range from a few centimeters to less than 1 cm in less preferred cases. For certain applications, for example banknotes, the total target thickness can be quite small (on the order of 100 μm or less). In addition, the allowable thickness variation (relief) accommodates mechanical constraints (weak spots associated with thinner areas) and operational considerations (e.g., when stacking banknotes, thicker portions of the banknotes). smaller for a variety of reasons, including the bill pile up and complicating handling and storage). Typically, for banknotes with a total thickness of about 100 μm, the target thickness for the relief pattern of the optical security elements contained in the banknote can be about 30 μm. For a credit or ID card with a thickness of about 1 mm, the target thickness for the relief pattern of the optical security element contained in this credit/ID card is less than about 400 μm, preferably less than or equal to about 250 μm.

Furthermore, the light source distance and image distance are generally limited to tens of centimeters by user convenience. Notable exceptions are daylights, or ceiling-mounted spotlights, but these are less readily available in certain circumstances. Also, the ratio d _s /d _i of the two distances is usually greater than 5 and up to 10 in order to obtain sharper (and better contrast) images that are more easily recognizable. In addition, the ratio d _s /d _i of 5 or more and the light source S preferably being point-like (e.g. conventional cell phone lighting LEDs), make the light source practically "at infinity". , and only projection planes at approximately the focal length from the optical security element are suitable for clearly viewing the projected caustic pattern. As a result, good visual observation conditions by the user do not require an overly strict relative spatial arrangement of the light source, the optical security element and the user's eye.

Generally, thickness and relief are the most important parameters. Considering an arbitrary target image (reference pattern) and optical geometry (i.e. geometrical conditions for illumination/observation of the projected caustic pattern) relief patterns for which the calculated optical surface is below a predetermined limit There is no guarantee that you will have In fact, in the general case the opposite can occur. This is especially true of the stringent constraints imposed on the optical security elements mentioned above. Considering that numerical simulations for optimizing optical surfaces are expensive in terms of time and resources, and excessive trial-and-error is not a viable option, useful results can be obtained on the first try, or at least marginally. It is highly desirable to be able to obtain reliably in only a reasonable number of trials. It is also highly desirable to have an unlimited choice of target images, since not all target images are compatible with shallow depth smooth relief patterns.

After much testing, it was found that this could be achieved by careful selection of the optical geometry, especially the target caustic pattern, given the depth constraints. Consider the following parameters (see Figure 1):
Image distance: d _i
Light source distance: d _s
Area of light deflection surface (cross-sectional area): A
The illuminance delivered by the projection surface from the light deflection surface upon illumination of the optical security element by the light source S: E _A ; over the area corresponding to the projection of the cross-section of the light deflection surface (i.e. its geometric shadow) means that the illuminance delivered to the projection plane when averaged has an average value equal to E _A Target relief pattern (maximum) depth: δ
Refractive index of the optical security element: n (for refractive light deflecting surfaces).

An optimized selection of the target caustic image, which makes it possible to provide a simple optical security element having a light deflection surface with a corresponding relief pattern in depth δ, is then "infinity". (that is, in practice, for d _s >>d _i , at least d _s ≧5d _i ), the relation α ₀ =2π(n−1) for a refractive optical element of refractive index n Any circular _{area α 1 ( α 1} <A), the quantity _Eα1 corresponding to the illumination averaged over a circular area _α1 at the projection plane (3) (preferably located at the focal plane of the optical security element) is given by the projection should meet the criteria.
E _α1 ≤ _EA (1/2 + α ₀ / α ₁ + (1/4 + α ₀ / α ₁ ) ^1/2 )

In practice, for a given value of _α1 (with the knowledge that the typical resolution length of the visible spectrum for the human eye is about 80 μm, at least the resolution when observing a caustic pattern at the screen is above the area), it is sufficient to scan with a viewing window of area _α1 over the area of the projected image and check that the corresponding illuminance E _α1 indeed satisfies the above projection criteria . Furthermore, in order to check whether the projection criteria are actually met, the candidate (target) relief pattern is effectively realized by machining the profile of the light deflection surface, performing the illumination of the optical element, and then It is not essential to scan the projected image on the screen with a viewing window of area _α1 . Distribution of light rays on the projection plane corresponding to given parameters (d _s , d _i , A, n (for refractive light-deflecting optical security elements), δ, E _A ) and a given target profile of the relief pattern A simple simulation (for example by ray tracing) of the scan operation with the test area α ₁ over , provides a high reliability check with respect to the projection criteria. Moreover, if the projection criterion is not met only in some specific sub-region of the projected image, it is fairly easy to locally adapt the corresponding part of the target profile to correct this defect (this is due to the corresponding equivalent to slightly modifying the reference pattern).

However, even such a simulation adaptation step (already much cheaper than conventional methods) can be eliminated. In fact, according to another aspect of the invention, a method is provided that allows the direct selection of a target relief pattern profile from a digital image of a reference pattern, from which it satisfies the projection criteria (for a given depth). A physical optical security element (having a light deflecting surface with a corresponding relief pattern) can easily be obtained. This method of designing a relief pattern of depth δ of a reflective light deflecting surface or a refractive transparent or partially transparent light deflecting surface of refractive index n to provide an optical security element that meets the above projection criteria. is based on specific digital image test criteria, which have been tested and proven to be very effective, and (with proper lighting/projection onto the screen) optical security The corresponding caustic pattern produced by the element is realized only for the digital image of the reference pattern to be reproduced.

Indeed, the course produced by an optical security element satisfying the projection criterion (and thus with given parameters d _s , d _i , n (for refractive optical security elements), δ, α ₀ , A, and E _A ) If the digital image of the candidate reference pattern to be reproduced on the projection plane by the tick pattern has a total number of pixels N _A , and the sum of each pixel value across the digital image has I _A , then N (N being an integer) in the digital image , and for each substantially circular region composed of 1≦N≦N _A ) pixels, the value I(N) of the sum of each pixel value of the N pixels in the circular region is equal to the value I _max (N)=N(I _A /N _A )(1/2+N ₀ /N+√(1/4+N ₀ /N)) where N ₀ is given by N _A (α ₀ /A) in the digital image It has been observed that a candidate reference pattern is convenient for effectively designing an optical security element capable of meeting projection criteria.

A candidate digital image of a reference pattern is scanned in a variable-sized viewing window of N pixels, where N varies up to N _A , such that the "window intensity" I(N) is a particular maximum value for the set of N pixels. The above selection test, which checks for less than I _max (N), can be fairly easily implemented in a processor (where the candidate digital images are stored in the processor's memory), and the corresponding execution of digital image processing is , gives a fast response to a complete scan of a digital image and thus greatly simplifies and accelerates the operation of designing an optical security element that meets projection criteria, i.e. the caustic pattern produced by this optical security element To enable an observer to easily determine whether an object marked with this optical security element is genuine.

Moreover, a further advantage of the above method is that the particular portion of the candidate digital image of the reference pattern that does not satisfy the selection test I(N)<I _max (N) (for some value(s) of N) is It's fairly easy to fix. This modifies the pixel values of a set of N pixels within said particular portion of the digital image and modifies the intensity I′(N) accordingly (i.e. It is sufficient that the sum of the modified pixel values) pass the selection test. This modification can also be easily implemented in a processor where the initial candidate digital images are stored in memory. Accordingly, the present invention provides a transformed digital reference pattern that meets the requirement on the selection test I(N)<I _max (N) for at least some values of N between 1 and N _A , allows easy adaptation of a given candidate digital reference pattern. For example, as shown in FIGS. 2A-2E and 3A-3E, the parameter values are A=1 cm×1 cm, d _s =30 cm, d _i =4 m, (maximum depth) δ=30 μm, and n=1. With .5, the scan windows used to test the digital images can each include some multiple of _n0 (this may be related to the resolution of the image, e.g. N _A (α ₀ /A)≈0.038 N _A , which may correspond to a fraction of the number of pixels given by A, which corresponds to a (substantially) circular area in pixels, and the target corresponding to the parameter α ₀ related to the optical security element). Appropriate boundary conditions, such as a reflective boundary condition, can be imposed to extend the scan to the edge of the image, where the image extends to the edge by imposing a mirror symmetry of extension on the edge. extended beyond FIG. 2 shows the reference pattern on a candidate digital image representing the number 100 (on a dark background) and having N _A =1024×1024 pixels. Figures 2A, 2B, 2C, 2D, and 2E show N= _n0 , _4n0 , _16n0 , _64n0 , and 256n, respectively, with _n0 = 314 (where _N0 = 3.9 x ¹⁰⁴ ). Shown are the results of scanning candidate digital images using scanning (circular) windows W1, W2, W3, W4, W5 of ₀ pixels, each scanned image in a normalized grayscale from 0 to I _max (N). (grayscale bars are shown on the right side of FIGS. 2A-E, pixel values range from 0 for black corresponding to I(N)=0 to white corresponding to I(N)=I _max (N) , up to 255). The physical (projected) image size is 10 mm x 10 mm, corresponding to a pixel size of approximately 0.0098 mm. In areas corresponding to respective numbers of numerals 100 in FIG. 2D with respective defined perimeters represented in dashed outline, and in the center of FIG. 2E with defined perimeters represented in dashed outline. The scanned image in FIG. 2D hardly passed the selection test I(N)<I _{max (N) because the value of I(N) reaches I max (} N) throughout the part (appearing as white areas). , the image of FIG. 2E does not satisfy the selection test I(N)<I _max (N). The candidate image of FIG. 2 is therefore not suitable for obtaining an optical security element with a low relief (here δ=30 μm).

In contrast, the reference pattern on the candidate digital image representing the number 100, but with additional lines in the background (i.e., guilloche intaglio-like) as shown in FIG. 3 (using the same parameter values as in FIG. 2). pattern) is drawn successfully satisfies the selection test I(N)<I _max (N). This is evident from FIGS. 3A, 3B, 3C, 3D and 3E, which are scans (circular) of N=n ₀ , 4n ₀ , 16n ₀ , 64n ₀ and 256n ₀ pixels respectively. The results of scanning candidate digital images using a window are shown, each scanned image being represented by a normalized grayscale from 0 to I _max (N) (grayscale bars are shown on the right side of FIG. 3D). and the pixel values are from 0 for black to 255 for white). There are no areas where I(N) reaches the value I _max (N) (there are no parts of the image with defined perimeters surrounding white areas).

According to a preferred variant of the invention, instead of modifying pixel values in specific parts of the candidate digital image of the reference pattern that do not satisfy the selection test, a filtering operation is applied globally to the candidate image to reduce the image contrast. , where the parameters of the filter are adapted to the projection criterion (eg, a high-pass filter with cut-off frequency is adapted to the reference pattern) has been successfully tested.

Therefore, the novel method described above enables efficient selection of convenient reference patterns as follows. scanning a digital image of this reference pattern according to certain selection criteria related to image pixel values, or modifying unsuitable candidate reference patterns to arrive at a suitable pattern, calculating a corresponding relief pattern, can be reproduced by appropriately machining the profile of the surface of the optical material substrate to form the light deflection surface of the optical element, and still meet the projection criteria despite the very low relief depth and smaller size. Able to reach optical security elements.

Therefore, according to the present invention, a relief pattern of given (very low) depth is designed to form a light deflection surface in an optical material substrate, (parameters d _s , d _i , (maximum depth ) corresponding to a set of values for δ, A, E _A , n (for refractive optical security elements) providing an optical security element capable of meeting the above projection criteria comprises the following steps: include.
i) selecting a convenient reference pattern by scanning a digital image of this reference pattern according to a specific selection criterion I(N)<I _max (N) (1≦N≦N _A ), wherein I _max (N)=N(I _A /N _A )(1/2+N ₀ /N+√(1/4+N ₀ /N)) where N ₀ is given by N _A (α ₀ /A) in the digital image a step, which is the number of pixels taken;
ii) calculating a relief pattern of depth no greater than δ corresponding to the reference pattern selected in step i);
iii) processing the surface of the optical material substrate to form a light deflecting surface having a relief pattern of depth values calculated in step ii); The resulting optical security element can be used for visual authenticity purposes.

FIG. 4A shows a very thin optical security element (i.e. front Shows a photograph of the realization of the image transparency). The total depth of the optical security element is 100 μm and its area A is 1 cm ² . The refractive material of the foil has a refractive index n of about 1.5 and is made of polyester. The refractive index of the resin used to form the relief pattern is also approximately 1.5. Also shown on the screen (see also FIG. 4B) is the projected caustic pattern (rear view). The reference pattern is that of FIG.

FIG. 4B is a photograph of the caustic pattern projected by the optical security element of FIG. 4A. Here the point light source is an LED at a distance d _s =30 cm from the light deflection surface and the flat screen onto which the caustic pattern is projected is at a distance d _i =40 mm. The caustic pattern is an intaglio pattern of the reference pattern of FIG. 3 and adequately reproduces the number 100 pattern.

FIG. 5 is an illustration of a projected caustic pattern corresponding to a reference pattern showing a portrait of George Washington from a 30 μm deep relief pattern, satisfying the projection criteria of the present invention, with good contrast and It shows that you can project very small details that you can see.

The above-disclosed subject matter is to be considered illustrative rather than limiting, and serves to provide a better understanding of the invention as defined by the independent claims.

Claims (7)

a relief pattern (2 ) of a reflective light-deflecting surface of the optical security element (1) or of a refractive transparent or partially transparent light-deflecting surface of refractive index n, with a depth no greater than a value δ which is no greater than 250 μm; A method for designing, wherein said relief pattern (2) deflects incident light received from a point light source (S) at a distance _ds from said light deflection surface and projects a projected image at a distance ds from said light deflection surface. a flat projection plane located at d _i , adapted to form in a projection plane (3) located in the focal plane of said optical security element (1), said point light source (S) Thus, when illuminating an area of value A of said relief pattern (2) and upon delivery of illumination value E _A by said optical security element (1) to said projection plane (3), said projection plane (3) The average illumination value E _α1 over a circular area of value α ₁ selected within the region of said projection image at is given by the projection criterion E _α1 ≤ E _A (1/2+α ₀ /α ₁ +√(1/4+α ₀ /α ₁ )) with a scaling area parameter α ₀ =4πd _i δ for the reflective light-deflecting surface or α ₀ =2π(n−1)d _i δ for the refractive light-deflecting surface and α ₁ is smaller than the area value A, the value of d _i is 30 cm or less, and the value of the ratio d _s /d _i is at least 5 or more,
a) selecting a digital image of a reference pattern (5) to be reproduced on said projection plane (3) by said projection image , said digital image having a total number of pixels _NA and spanning said digital image; The sum of all pixel values is I _A , and for each circular region of N (where N is an integer, 1≦N≦N _A ) pixels in said digital image, said N pixels in said circular region The value I(N) of the sum of each pixel value of the pixel is less than the value I _max (N)=N(I _A /N _A )(1/2+N ₀ /N+√(1/4+N ₀ /N)) said selection being made by checking that N ₀ is the number of pixels in said digital image given by N _A (α ₀ /A);
b) calculating a relief pattern (2) of depth less than or equal to δ corresponding to said reference pattern (5) in said digital image selected in step a);
c) processing the surface of an optical material substrate to form a light deflection surface that reproduces said relief pattern (2) calculated in step b), an optical security element comprising said processed light deflection surface ( 1) obtaining;
A method comprising: said step a) of selecting a digital image of a reference pattern (5) selects candidate digital images of said reference pattern (5), some of which do not satisfy the selection criterion that I(N) is less than _Imax (N); a further step of modifying, said further step providing said portion of said candidate digital image with adapted pixel values to comply with said selection criteria for any N, 1≤N≤N _A . 2. The method of claim 1 , wherein the method is performed by adapting the pixel values within the portion of the candidate digital image to provide a modified candidate digital image for selection. 3. The method of claim 2 , wherein the pixel values of the candidate digital image are adapted by filtering the candidate image with a filter to reduce image contrast. The method according to any one of claims 1 to 3 , wherein said processing of said surface of said optical material substrate comprises any one of ultra-precision processing, laser ablation and lithography. A method according to any one of claims 1 to 4 , further comprising that the light deflecting surface to be machined is a master light deflecting surface used to manufacture replicas. 6. The method of claim 5 , further comprising replicating the machined light-deflecting surface on a substrate. 7. A method according to claim 5 or 6 , wherein replication comprises one of UV casting and embossing.

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